1. Field of the Invention
The present invention relates to a sheet finisher mounted on or operatively connected to a copier, printer or similar image forming apparatus for sorting, stacking, stapling, punching, positioning, folding or otherwise finishing a sheet or sheets carrying images thereon, and an image forming system consisting of the sheet finisher and an image forming apparatus.
2. Description of the Background Art
Today, a sheet finisher for the above application is extensively used and located downstream of an image forming apparatus for finishing sheets, or recording media, in various ways. An advanced finisher recently proposed has multiple functions including a center stapling function and a folding function in addition to an edge stapling function. Japanese Patent Laid-Open Publication No. 2001-19269, for example, discloses a sheet finisher including a roller pair configured to fold a sheet stack at the center while conveying the sheet stack via its nip.
Japanese Patent Laid-Open Publication Nos. 7-48062 and 2000-153947, for example, each disclose a sheet finisher in which edge stapling and center stapling are effected independently of each other with a sheet path being switched at the inlet of the finisher. Although this type of sheet finisher can be easily constructed into a unit and can adapt to a less-option configuration, it is not desirable in the cost aspect because its functions overlap each other. Further, in a center staple mode, the sheet finisher performs folding of a sheet stack at the same position as positioning and stapling, so that a sheet stack of the next job cannot be brought to the center stapling position until the folding of the previous job completes. This prevents productivity from being enhanced.
In light of the above, Japanese Patent Laid-Open Publication Nos. 2000-11886 and 7-187479, for example, each teach a sheet finisher including a staple tray or processing tray inclined such that its downstream side in the direction of sheet feed is higher in level than the upstream side. A sheet stack is positioned and stapled on such a staple tray in either one of an edge staple mode and a center staple mode and then switched back to be conveyed to another station, which is assigned to folding. More specifically, the stapled sheet stack is conveyed in a direction opposite to a direction in which a sheet stack stapled at its edge is to be discharged. The folding station arranged independently of the stapling station enhances productivity and minimizes an increase in cost ascribable to overlapping mechanisms. However, a fold tray located at the folding station must be configured long enough to enhances productivity. As a result, the staple tray positioned above the fold tray and the fold tray are contiguous with each other in a “<” configuration, making the sheet finisher bulky. This cannot meet the increasing demand for space saving.
For size reduction, Japanese Patent Laid-Open Publication No. 2000-63031, for example, proposes a sheet finisher constructed to fold a sheet stack extending over two processing trays. This construction, however, cannot enhance productivity.
Japanese Patent Laid-Open Publication Nos. 11-286368 and 2000-86067 each propose a sheet finisher in which a fold roller is positioned slightly above the intermediate portion of a fold tray so as to directly fold a sheet stack and then drive it out of the finisher, thereby implementing the shared use of a processing tray and a short conveyance path. Such a sheet finisher, however, not only fails to enhance productivity, as stated earlier, but also is large size because the fold roller is positioned above the inclined tray.
Of course, a sheet finisher with a single function, i.e., a center stapling function, as disclosed in Japanese Patent Laid-Open Publication No. 9-183558, cannot meet the needs on today's market.
Generally, in a staple mode available with a sheet finisher, it is a common-practice to position consecutive sheets on a position tray, staple the resulting sheet stack with stapling means, and then convey the stapled sheet stack to a tray located at the most downstream portion of the sheet finisher. In a center staple mode, a sheet stack stapled at the center is conveyed to a folding section and then conveyed to the above tray. This type of sheet finisher includes a plurality of paths each being assigned to a particular mode and path switching means for selecting one of the paths matching with a mode selected.
When the sheet finisher with the folding function stated above conveys a sheet stack to a folding station, the sheet stack is apt to become loose if conveyed at high speed although the speed may allow a stapled sheet stack to be surely conveyed. The loose sheet stack cannot be stapled in a neat configuration. However, if the conveying speed is lowered, then the next sheet stack (job) cannot be received. This lowers CPM and therefore requires the productivity of the image forming apparatus to be lowered. That is, how high the operation speed of the image forming apparatus may be, the productivity of the image forming apparatus is limited by the ability of the sheet finisher.
Assume that the path switching means is operated when a job for outputting a desired number of sets (copies) of copies of documents or outputting a plurality of booklets is to be executed. For example, assume that in a center staple mode the path switching means selects a path for conveying a sheet stack downward from a staple tray instead of a path for conveying it upward from the staple tray. Then, the path switching means catches a sheet entering the staple tray and causes it to jam the path or to crease or otherwise deform. Further, if the path switching means is so positioned as to select the downward path when a sheet stack jams the path at a branch portion, it is difficult for the operator to remove the jamming sheet stack.
Moreover, in the case where a sheet stack includes a cover or a slip sheet different in kind and size from the other sheets, a roller or a projection included in the path switching means is likely to catch the sheet stack and damage it. More specifically, the size of a sheet varies when it is passed through a fixing section in accordance with the degree of moisture absorption.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 10-59610, 10-181990, 10-218475, 2000-72320, 2000-118860, 2000-143081 and 2000-68577.